The present invention relates to a method for coating an article with plastic. The invention is especially directed towards the coating of shafts with plastic, and the new method makes use of an ultrasound source. The method entails a favourable production solution for depositing plastic on metal shafts in a more accurate way than with today""s solutions. The metal shafts are preferably made of steel, aluminium or manganese, but the invention is not limited to coating these metals.
The products that are coated by means of the inventive method may, e.g., by sliding surfaces on shafts that are used as steering wheel columns in cars. Great precision is required here to be able to move the steering wheel back and forth and to absorb impact in the event of a collision.
Today, there are several known methods of coating a shaft.
One previously known method is to use injection moulding, where a shaft is placed inside a mould, and plastic is injected into the mould at high pressure.
A second method is to use an extruded plastic sleeve which has been cut to a suitable length and is of the right shape and size. The plastic sleeve is passed onto the shaft, heat is subsequently applied to shrink the plastic and then ultrasound to calibrate the plastic on the shaft against the sleeve. By calibrating the shaft against the sleeve the proportions are adjusted so they are within the tolerances when the shaft is passed into the sleeve at the same time as ultrasound is applied. This is done to obtain better adaptation of the plastic coating and the sleeve than earlier, as the sleeves have different shapes and sizes.
Another method is to use a plastic powder which is applied to a heated metal. The temperature in the process must be higher than the melting point of the plastic. In this way the powder will form a thin layer of plastic on the metal.
The present invention is based on known technology and exploits it further to make products having a greater accuracy within specified tolerance limits. With the inventive method it is possible to operate easily within tolerance limits of from +/xe2x88x923/100 to +/xe2x88x923/1000.
According to the invention, the methods comprises depositing plastic on a shaft, and then moving a metal sleeve over the shaft and plastic sleeve, while ultrasound is employed to calibrate the plastic on the shaft.
The present invention is directed towards a method for applying a coating of plastic on a metal shaft, wherein:
(i) plastic is applied onto the shaft by means of injection moulding;
(ii) an extruded plastic sleeve cut to the right length is placed on the shaft;
(iii) the plastic is sprayed onto the shaft with the aid of a sprayer;
(iv) the plastic is applied by lowering the shaft into a plastic solution; or
(v) plastic is applied in powder form to the heated metal of the shaft;
characterised in that after the plastic has been applied, a metal sleeve is moved relative to the shaft and the plastic applied thereto at the same time as the whole unit is supplied with ultrasound to calibrate the plastic sleeve on the shaft.